In general, the angular wavenumber k (i.e. the magnitude of the wave vector) is given by

k = \frac{2\pi}{\lambda} = \frac{2\pi\nu}{v_\mathrm{p}}=\frac{\omega}{v_\mathrm{p}}
where \nu is the frequency of the wave, \lambda is the wavelength, \omega = 2\pi\nu is the angular frequency of the wave, and vp is the phase velocity of the wave. The dependence of the wavenumber on the frequency (or more commonly the frequency on the wavenumber) is known as a dispersion relation.